Method for protecting a ballast from an output ground-fault condition

ABSTRACT

A method ( 700 ) for protecting a ballast from an output ground-fault condition includes the steps of applying power to the ballast (step  710 ), providing a startup delay period for a DC-to-DC converter (step  730 ), monitoring a voltage across a DC blocking capacitor (step  740 ), preventing startup of the DC-to-DC converter in response to the voltage across the DC blocking capacitor being greater than a threshold value during the startup delay period (step  750 ), and allowing startup of the DC-to-DC converter in response to the voltage across the DC blocking capacitor being less than the threshold value throughout the startup delay period (step  760 ). Preferably, the steps of monitoring (step  740 ), preventing (step  750 ), and allowing (step  760 ) are implemented by a microcontroller, and the DC-to-DC converter is preferably implemented as either a Sepic converter or a buck converter.

FIELD OF THE INVENTION

The present invention relates to the general subject of circuits forpowering discharge lamps. More particularly, the present inventionrelates to a method for protecting a ballast from damage in the event ofan output ground-fault.

BACKGROUND OF THE INVENTION

A number of existing electronic ballasts have non-isolated outputs. Suchballasts typically include circuitry for protecting the ballast inverterfrom damage in the event of a lamp fault condition (e.g., removal orfailure of a lamp).

Occasionally, the output wiring of a ballast [i.e., the wires thatconnect the ballast to the lamp(s)] becomes shorted to earth ground viathe lighting fixture. Such a condition can arise, for example, due tothe output wires becoming loose or pinched. For ballasts withnon-isolated outputs, if the inverter begins to operate while an earthground short is present at one or more of the output wires, a large lowfrequency (e.g., 60 hertz) current may flow through the invertertransistors and cause them to fail.

U.S. Pat. No. 6,657,400 B2 (entitled “Ballast with Protection Circuitfor Preventing Inverter Startup During an Output Ground-Fault Condition”and assigned to the same assignee as the present invention) discloses aballast that includes an output ground-fault protection circuit. Theground-fault protection circuit that is disclosed in the U.S. Pat. No.6,657,400 is well suited for many ballasts, but has the drawback ofrequiring additional discrete circuitry in order to provide outputground-fault protection.

In recent years, it has become increasingly common for electronicballasts to include a programmable microcontroller that coordinates andcontrols multiple functions (e.g., lamp fault protection) within theballast. For such ballasts, a need exists for a ground-fault protectionapproach that can be realized with little or no additional circuitry. Aballast that includes such a ground-fault protection approach wouldrepresent a significant advance over the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematic of a ballast with outputground-fault protection, in accordance with the preferred embodiments ofthe present invention.

FIG. 2 is a detailed electrical schematic of a ballast with outputground-fault protection, in accordance with a first preferred embodimentof the present invention.

FIG. 3 is a detailed electrical schematic of a ballast with outputground-fault protection, in accordance with a second preferredembodiment of the present invention.

FIG. 4 is a flowchart that describes a method for protecting a ballastfrom an output ground-fault condition, in accordance with the preferredembodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 describes a ballast 10 for powering at least one gas dischargelamp 40. Ballast 10 comprises an EMI filter 100, a full-wave rectifier200, a DC-to-DC converter 300, an inverter 400, an output circuit 500,and a microcontroller 600.

EMI filter 100 includes input terminals 102,104 adapted to receive aconventional source of alternating current (AC) voltage 50, such as 120volts rms at 60 hertz. Full-wave rectifier 200 is coupled to EMI filter100. DC-to-DC converter 300 is coupled to full-wave rectifier 200.Inverter 400 is coupled to DC-to-DC converter 300. Output circuit 500 iscoupled to inverter 400, and includes output terminals 502,504,506,508adapted for connection to gas discharge lamp 40. Microcontroller 600 iscoupled to DC-to-DC converter 300, inverter 400, and output circuit 500.

During operation, DC-to-DC converter 300 receives a full-wave rectifiedvoltage from full-wave rectifier 200 and provides a substantially directcurrent (DC) output voltage to inverter 400 via terminals 402,404.DC-to-DC converter 300 has a non-operating mode (during which the DCoutput voltage is substantially zero, which occurs prior to startup ofthe DC-to-DC converter) and an operating mode (during which the DCoutput voltage is substantially greater than zero, which occurs afterstartup of the DC-to-DC converter). In response to an outputground-fault condition wherein at least one of output terminals502,504,506,508 is shorted to earth ground, microcontroller 600 directsDC-to-DC converter 300 to remain in the non-operating mode. By forcingDC-to-DC converter 300 to remain in the non-operating mode if an outputground-fault condition is present, microcontroller 600 protects inverter400 from damage that would otherwise occur. Preferably, microcontroller600 is realized by an integrated circuit (IC), such as a ST7LITE1Bmicrocontroller IC manufactured by ST Microelectronics, along withassociated peripheral circuitry.

Turning now to FIG. 2, in a first preferred embodiment of the presentinvention, ballast 20 includes a DC-to-DC converter 300 that isimplemented as a Sepic converter. Sepic converter 300 comprises a firstinductor 310, an electronic switch 320 (preferably implemented as aN-channel field effect transistor), a first capacitor 325, a drivecircuit 330, a second inductor 340, a diode 350, and a second capacitor360. Further details regarding the construction and theory of operationof Sepic converter 300 are well known to those skilled in the art ofpower supplies and electronic ballasts, and thus will not be furtherelaborated upon herein. Nevertheless, for purposes of understanding thepresent invention, it is important to appreciate that, while in thenon-operating mode (i.e., during which time drive circuit 325 does notcommutate electronic switch 320), the output voltage (provided betweenterminals 402,404) of Sepic converter 300 is approximately zero. When anoutput ground-fault condition is detected following application of powerto ballast 20, microcontroller 600 directs Sepic converter 300 to remainin the non-operating mode, thereby ensuring that substantially zerovoltage is supplied to inverter 400. This protects inverter 400 from thedamage that would otherwise occur due to presence of a ground-fault atany of output terminals 502,504,506,508.

As described in FIG. 2, EMI filter 100 includes magnetically coupledinductors 120,122, an X-capacitor 130, and a Y-capacitor 140 having oneend that is coupled to earth ground 60. Full-wave rectifier 200 includesa diode bridge 210 and a capacitor 220. Inverter 400 is preferablyimplemented as a half-bridge type inverter that includes first andsecond inverter switches 410,420 (preferably realized by N-channelfield-effect transistors) and an inverter drive circuit 430 thatprovides substantially complementary commutation of inverter switches410,420. Output circuit 500 is preferably implemented as a seriesresonant type output circuit comprising first, second, third, and fourthoutput terminals 502,504,506,508, a resonant inductor 510, a resonantcapacitor 520, a direct current (DC) blocking capacitor 530, a firstfilament heating circuit comprising a first winding 512 (preferably,first winding 512 is magnetically coupled to resonant inductor 510) anda first capacitor 522, a second filament heating circuit comprising asecond winding 514 (preferably, second winding 514 is magneticallycoupled to resonant inductor 510) and a second capacitor 524, andfilament path resistors 540,542,544,546. First and second outputterminals 502,504 are adapted for connection to a first filament 42 oflamp 40. Third and fourth output terminals 506,508 are adapted forconnection to a second filament of lamp 40. DC blocking capacitor 530 iscoupled between fourth output terminal 508 and circuit ground 70.

As known to those skilled in the art of power supplies and electronicballasts, output circuit 500 may be modified in certain well-known ways(which differ from that which is described in FIG. 2) withoutsubstantially affecting the desired operation of ballast 20. Forexample, the lower end of resonant capacitor 520 may alternatively becoupled directly to circuit ground 70 (instead of being coupled tofourth output terminal 508 and the top of DC blocking capacitor 530). Asa further example, capacitors 522,524 in the first and second filamentheating circuits may be replaced with diodes. Various othermodifications to output circuit 500 will be apparent to those skilled inthe art of power supplies and electronic ballasts.

Referring again to FIG. 2, microcontroller 600 includes a startup input602, a detection input 604, a first control output 606, and a secondcontrol output 608. It should be appreciated that, in an actual ballast,microcontroller 600 will include additional inputs and outputs (for thesake of clarity, those inputs and outputs are not shown or describedherein) for implementing other ballast control functions, such as lampfault protection, control of lamp current or power, and so forth. Asdescribed in FIG. 2, startup input 602 is coupled to the output offull-wave rectifier 200 via resistors 80,82. During operation, startupinput 602 receives voltage/current necessary for operatingmicrocontroller 600. Detection input 604 is coupled to DC blockingcapacitor 530 and fourth output terminal 508. During operation,detection input 604 allows microcontroller 600 to monitor a voltage,V_(BLOCK), across DC blocking capacitor 530. The magnitude of V_(BLOCK)prior to startup of Sepic converter 300 indicates whether or not anoutput ground-fault condition is present. More specifically, if avoltage that is greater than a predetermined threshold value (e.g., 100millivolts) is present at detection input 604 prior to startup of Sepicconverter 300, then an output ground-fault condition is deemed to bepresent; in response, microcontroller 600 directs Sepic converter 300 toremain in the non-operating mode. In this way, microcontroller protectsinverter 400 from being damaged when an output ground-fault is present.Conversely, if a voltage that is less than the predetermined thresholdvalue (e.g., 100 millivolts) is present at detection input 604throughout the period prior to startup of Sepic converter 300, then anoutput ground-fault condition is deemed to not be present; in response,microcontroller 600 allows Sepic converter 300 to enter the operatingmode.

For practical reasons, it is preferred that the predetermined thresholdvalue be a small nonzero voltage that is on the order of about 100millivolts or so. This is desirable in order to provide some degree ofimmunity to possible electrical noise (that might otherwise falselyindicate an output ground-fault condition).

The detailed operation of ballast 20 is now explained with reference toFIG. 2 as follows.

During normal operation, when no output ground-fault is present, ballast20 operates in the following manner. When power is initially applied toballast 20 (at t=0), DC-to-DC converter 300, inverter 400, andmicrocontroller 600 are initially off. Within a short period of timefollowing initial application of power to ballast 20, microcontroller600 turns on due to the voltage provided to startup input 602. At thatpoint, DC-to-DC converter 300 and inverter 400 are still off. DC-to-DCconverter 300 will remain off (i.e., in the non-operating mode) for apredetermined startup delay period (i.e., 0<t<t₁). With microcontroller600 turned on, microcontroller 600 monitors (via detection input 604)the voltage, V_(BLOCK), across DC blocking capacitor 530. Because nooutput ground-fault is present, and because both DC-to-DC converter 300and inverter 400 are not yet operating, V_(BLOCK) will be approximatelyzero during this time. Accordingly, at the end of the predeterminedstartup delay period (i.e., t=t₁), microcontroller 600 will allowDC-to-DC converter 300 to start in a normal manner, at which pointDC-to-DC converter 300 will provide a nonzero output voltage betweenterminals 402,404. Inverter 400 subsequently starts and proceeds toprovide, via output circuit 500, voltages for preheating lamp filaments42,44, a high voltage for igniting lamp 40, and a magnitude-limitedcurrent for operating lamp 40 after ignition.

If, on the other hand, an output ground-fault condition is present(i.e., at least one of output terminals 502,504,506,508 is shorted toearth ground), ballast 20 operates in the following manner. When poweris initially applied to ballast 20 (at t=0), DC-to-DC converter 300,inverter 400, and microcontroller 600 are initially off. Within a shortperiod of time following initial application of power to ballast 20,microcontroller 600 turns on due to the voltage provided to startupinput 602. At that point, DC-to-DC converter 300 and inverter 400 arestill off. DC-to-DC converter 300 will remain off; (i.e., in thenon-operating mode) for a predetermined startup delay period (i.e.,0<t<t.sub.1). With microcontroller 600 turned on, microcontroller 600monitors (via detection input 604) the voltage, V.sub.BLOCK, across DCblocking capacitor 530. With an output ground-fault condition present, alow frequency (e.g., 60 hertz) current flows up from earth ground 60 tothe shorted output terminal (502 or 504 or 506 or 508), through one orboth lamp filaments 42,44 (depending on which output terminal is shortedto earth ground), through filament path resistors 544,546 (if theground-fault is present at output terminal 502 or 504), through DCblocking capacitor 530, and into circuit ground 70. The resulting lowfrequency current that flows in the event of an output ground-faultcauses a nonzero voltage that is substantially greater than apredetermined threshold value (e.g., 100 millivolts) to develop acrossDC blocking capacitor 530. That nonzero voltage is detected bymicrocontroller 600, which responds by directing Sepic converter 300 toremain off (i.e., in the non-operating mode). In this way, ballast 20 isprotected from the damage (e.g., destruction of inverter transistors410,420) that would otherwise occur due to an output ground-faultcondition.

As described herein, microcontroller 600 is responsive to protectballast 20 from a ground-fault condition at either of output terminals502,504,506,508. However, it should be appreciated that, in the absenceof appropriate protection, a ground-fault at output terminal 502 or 504would be potentially more destructive than a ground-fault at outputterminal 506 or 508.

Turning now to FIG. 3, in a second preferred embodiment of the presentinvention, ballast 30 includes a DC-to-DC converter that is implementedas a buck converter 300′. Buck converter 300′ comprises an inductor 310,an electronic switch 320 (preferably realized by a N-channelfield-effect transistor), a drive circuit 330′, a diode 350, and acapacitor 360. Details regarding the construction and theory ofoperation of buck converter 300′ are well known to those skilled in theart of power supplies and electronic ballasts, and thus will not befurther elaborated upon herein. However, for purposes of understandingthe present invention, it is important to appreciate that, while in thenon-operating mode (i.e., during which time drive circuit 325′ does notcommutate electronic switch 320), the output voltage (provided betweenterminals 402,404) of buck converter 300′ is approximately zero. When anoutput ground-fault condition is detected following application of powerto ballast 20, microcontroller 600 directs buck converter 300 to remainin the non-operating mode, thereby ensuring that substantially zerovoltage is supplied to inverter 400. This protects inverter 400 from thedamage that would otherwise occur due to the presence of a ground-faultat any of output terminals 502,504,506,508.

In the second preferred embodiment, as described in FIG. 3, thepreferred structures for EMI filter 100, full-wave rectifier 200,inverter 400, output circuit 500, and microcontroller 600 are identicalto that which was previously described in connection with the firstpreferred embodiment (i.e., ballast 20) described in FIG. 2. Moreover,the detailed operation of ballast 30 is essentially the same as thatwhich was previously described with reference to ballast 20 (FIG. 2).

FIG. 4 describes a method, for a ballast that includes a DC-to-DCconverter and a direct current (DC) blocking capacitor, for protectingthe ballast from an output ground-fault condition. The method 700comprises the steps of: (1) applying power to the ballast (step 710);(2) activating a microcontroller (step 720); (3) providing a startupdelay period (0<t<t₁) for the DC-to-DC converter (step 730); (4)monitoring a voltage, V_(BLOCK), across the direct current (DC) blockingcapacitor during the startup delay period (step 740); (4) in response toV_(BLOCK) being greater than a predetermined threshold value (e.g., 100millivolts) during the startup delay period, preventing startup of theDC-to-DC converter (decision block 742 and step 750); (5) in response toV_(BLOCK) being less than the predetermined threshold value throughoutthe startup delay period, allowing startup of the DC-to-DC converter att=t₁ (decision blocks 742,744 and step 760). In accordance with thefirst and second preferred embodiments described herein, the steps ofmonitoring (step 740), preventing (step 750), and allowing (step 760)are executed via the microcontroller, and the DC-to-DC converter ispreferably implemented as either a Sepic converter or a buck converter.Moreover, it is preferred that the predetermined threshold value be asmall nonzero voltage that is on the order of about 100 millivolts orso, in order to provide some degree of immunity to electrical noise.

Although the present invention has been described with reference tocertain preferred embodiments, numerous modifications and variations canbe made by those skilled in the art without departing from the novelspirit and scope of this invention. For example, although the presentdescription is directed to ballasts 10,20,30 that power a single gasdischarge lamp 40, the principles of the present invention are readilyextended and applied to ballasts that power multiple gas dischargelamps. Additionally, the DC-to-DC converter is not limited to a Sepic orbuck converter, but may be implemented by any other type of converter(e.g., a flyback converter or a buck+boost converter) that provides asubstantially zero output voltage prior to startup.

1. A method for protecting a ballast from an output ground-faultcondition, the method comprising the steps of: applying power to theballast; providing a startup delay period for a DC-to-DC converter;monitoring a voltage across a direct current (DC) blocking capacitor,wherein the DC blocking capacitor is operably coupled in series with theat least one gas discharge lamp; preventing startup of the DC-to-DCconverter, in response to the voltage across the DC blocking capacitorbeing greater than a predetermined threshold value during the startupdelay period; and allowing startup of the DC-to-DC converter, inresponse to the voltage across the DC blocking capacitor being less thanthe predetermined threshold value throughout the startup delay period.2. The method of claim 1, further comprising the step of activating amicrocontroller, wherein the step of activating the microcontroller isexecuted between the steps of applying power to the ballast andproviding a startup delay period for a DC-to-DC converter.
 3. The methodof claim 2, wherein the step of monitoring a voltage across a DCblocking capacitor is executed via the microcontroller.
 4. The method ofclaim 3, wherein the steps of preventing startup of the DC-to-DCconverter and allowing startup of the DC-to-DC converter are executedvia the microcontroller.
 5. The method of claim 1, wherein the DC-to-DCconverter is one of: (i) a Sepic converter; and (ii) a buck converter.6. The method of claim 1, wherein the predetermined threshold value ison the order of about 100 millivolts.
 7. A method for protecting aballast for powering at least one gas discharge lamp from an outputground-fault condition, the method comprising the steps of: applyingpower to the ballast; activating a microcontroller; providing a startupdelay period for a Sepic converter; monitoring a voltage across a directcurrent (DC) blocking capacitor that is operably coupled in series withthe at least one gas discharge lamp; preventing startup of the Sepicconverter, in response to the voltage across the DC blocking capacitorbeing greater than a predetermined threshold value during the startupdelay period; and allowing startup of the Sepic converter, in responseto the voltage across the DC blocking capacitor being less than apredetermined threshold value throughout the startup delay period. 8.The method of claim 7, wherein the step of monitoring a voltage across aDC blocking capacitor is executed via the microcontroller.
 9. The methodof claim 8, wherein the steps of preventing startup of the Sepicconverter and allowing startup of the Sepic converter are executed viathe microcontroller.
 10. The method of claim 7, wherein thepredetermined threshold value is on the order of about 100 millivolts.11. A method for protecting a ballast for powering at least one gasdischarge lamp from an output ground-fault condition, the methodcomprising the steps of: applying power to the ballast; activating amicrocontroller; providing a startup delay period for a buck converter;monitoring a voltage across a direct current (DC) blocking capacitorthat is operatively coupled in series with the at least one gasdischarge lamp; preventing startup of the buck converter, in response tothe voltage across the DC blocking capacitor being greater than apredetermined threshold value during the startup delay period; andallowing startup of the buck converter, in response to the voltageacross the DC blocking capacitor being less than the predeterminedthreshold value throughout the startup delay period.
 12. The method ofclaim 11, wherein the step of monitoring a voltage across a DC blockingcapacitor is executed via the microcontroller.
 13. The method of claim12, wherein the steps of preventing startup of the buck converter andallowing startup of the buck converter are executed via themicrocontroller.
 14. The method of claim 11, wherein the predeterminedthreshold value is on the order of about 100 millivolts.